X-PressMatter Group' IHPP PAS

Soft  Matter  Blog

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 Publications of our Group

Caloric effects in liquid crystal-based soft materials

 With the increased environmental awareness, the search for environmentally friendlier heat-management techniques has been the topic of many scientific studies. The caloric materials with large caloric effects, such as the electrocaloric (EC) and elastocaloric (eC) effects, have increased interest due to their potential to realize new solid-state refrigeration devices. Recently, caloric properties of soft materials, such as liquid crystals (LCs) and LC elastomers (LCEs), are getting more of the focus of caloric materials investigations, stimulated by large caloric effects observed in these materials. Here, an overview of recent direct measurements of large caloric effects in smectic LC 14CB and main-chain LCEs is given. Specifically, high-resolution thermometric measurements revealed a large EC response in 14CB LC exceeding 8K. Such a large effect was obtained at a relatively moderate electric field of 30 kV cm−1 compared to solid EC materials. The work demonstrates that such a small field can induce the isotropic to smectic A phase transition in 14CB, releasing or absorbing relatively large latent heat that enhances the EC response. 

Impact of Weak Nanoparticle Induced Disorder on Nematic Ordering.

This paper presents an analyzed weak disorder-driven crossover behaviour of the nematic order parameter s on varying p deep in the nematic phase.  We used a Lebwohl-Lasher-type lattice semi-microscopic model. We set that NPs locally enforce a preferential orientational order, where the distribution of the corresponding easy directions is determined by the probability distribution P. We used a mean field–type approach and assumed that nematic spins locally align along the effective local field and derived the corresponding self-consistent equation for S. A simple cylindrically symmetric step-like shape of P yields the p-driven crossover behaviour which is observed experimentally. This shape in P roughly mimics cases where NPs generated preferred local orientations which are on average aligned along the global nematic order. Our analysis reveals that S(p) behaviour is relatively robust with respect to details describing P. Note that in the experiments P distribution is expected to quantitatively change on varying p. The modeling of this rearrangement mechanism is the topic of our next research.

Liquid crystals flow like isotropic liquids but they exhibit long–range order ofmolecules, typical for crystalline solids. This group of materials is probably best known for their use in displays (LCD, LED). As liquid crystals research is largely driven by the display industry, many papers focus on properties connected with the performance of these devices, including threshold voltage (minimum amount of voltage that is necessary to produce any molecular movement) or direct current (DC) electric conductivity (its source are ions, which cause image sticking). In our work, we chose a more fundamental approach; we show the temperature evolution of static and dynamic properties and describe them using theoretical models. 

Fluctuations-driven dielectric properties of liquid crystalline octyloxycyanobiphenyl and its nanocolloids 

 This report presents the pressure-temperature (p-T) plane of Bi2O3-Al2O3-SiO2 ternary system in the context of nanocrystallite formation from its amorphous state. The diagram was constructed through differential thermal analysis (DTA) performed in situ under high-pressure-high-temperature (HP-HT) conditions, with nitrogen serving as the pressurizing medium. Above the glass transition temperature Tg, a wide ultraviscous, supercooled liquid state spanning approximately 150 K is observed. Later heating transforms this state into nanocrystallites embedded within an amorphous matrix, thereby keeping distinctive structural characteristics even after the decompression process. The phase-related diagram covering the p-T plane is a fundamental prerequisite for the design of nanocrystallites within a glass matrix, essential for yielding different materials solely by compressing.

Nanocrystallization of Bi2O3 based system from the glassy state under high compression.

 The New Model discussion focuses on links between the unique properties of relaxor ceramics and the basics of Critical Phenomena Physics and Glass Transition Physics. It indicates the significance of uniaxiality for the appearance of mean-field type features near the paraelectric-to-ferroelectric phase transition. Pretransitional fluctuations, that are increasing up to the size of a grain, and leading to inter-grain, random local electric fields, are responsible for relaxor ceramics characteristics. Their impact yields the pseudospinodal behavior associated with “weakly discontinuous” local phase transitions. The emerging model redefines the meaning of the Burns temperature and polar nanoregions (PNRs). It offers a coherent explanation of “dielectric constant” changes with the “diffused maximum” near the paraelectric-to-ferroelectric transition, the sensitivity to moderate electric fields (tunability), and the “glassy” dynamics. 

Critical Insight into Pretransitional Behavior and Dielectric Tunability of Relaxor Ceramics. 

 Lyotropic liquid crystals are multicomponent systems of fundamental importance. They can also construct model structures for biological systems. The richness of their phases allows for the theoretical prediction of behaviour and the planning of m odel experiments enabling an approximation of the description of a living cell. In this work, we present experimental results for lyotropic liquid-crystalline lamellar phases, which may be used as a physical model of phospholipid bilayer. Dielectric measurements were performed under temperature and high-pressure conditions. Complex dynamics, dielectric response, static properties as well as electric conductivity were considered and analysed on the based on the critical phenomena theory. Other dielectric/electrostatic properties were also calculated and compared to evidence for biological systems.

Dielectric Study of Induced Phase Transitions in Lyotropic Liquid Crystals. 

The long-range supercritical changes of dielectric constant, resembling ones observed in the isotropic liquid phase of liquid crystalline compounds, are evidenced for linseed oil—although in the given case, the phenomenon is associated with the liquid–solid melting/freezing discontinuous phase transitions. This ‘supercriticality’ can be an additional factor supporting the unique pro-health properties of linseed oil. Broadband dielectric spectroscopy studies also revealed the ‘glassy’ changes of relaxation times, well portrayed by the ‘activated and critical’ equation recently introduced. In the solid phase, the premelting effect characteristic for the canonic melting/freezing discontinuous transition, i.e., without any pretransitional effect in the liquid phase, has been detected. It is interpreted within the grain model, and its parameterization is possible using the Lipovsky model and the ‘reversed’ Mossotti catastrophe concept. 

Supercriticality, Glassy Dynamics, and the New Insight into Melting/Freezing Discontinuous Transition in Linseed Oil.

To canonic features of melting/freezing discontinuous phase transitions, one can encounter the lack of pretransitional effects in the liquids state and weak premelting effects with non-defined temperature evolutions in the crystalline state. This report shows the evidence for long-range and critical-like pretransitional effects in liquid menthol for electrooptic Kerr effect (EKE) and nonlinear dielectric effect (NDE). These methods are inherently associated with a strong electric field.

The premelting effect, with well-defined temperature evolution, was evidenced in the crystalline state. Model explanations of these phenomena are proposed. For the premelting effect, the ‘reversed’ Mossotti-Catastrophe behavior is suggested.

Pretransitional and premelting effects in menthol.

 The report shows the implementation of the recently developed innovative method for determining the coexistence curve via the analysis of the fractional meniscus heights for unique mixtures showing the lower critical consolute temperature (LCT). The high-resolution experimental data enabled the precise analysis of the binodal curve regarding the order parameter and the diameter. To the best of the authors' knowledge, it is the first example of such analysis for the LCT type mixtures of limited miscibility. The authors stress the new concept for the simple and fast estimation of the critical concentration, finally solving the puzzling situation that appeared four decades ago after invalidating the Cailletet-Mathias law of rectilinear diameter. This report focuses on the binary mixtures characterized via the critical concentration. 

New insight into 3-picoline—deuterium oxide (D2O) mixtures of limited miscibility with the lower critical consolute temperature 

Of our interest are frustration-driven pattern generating mechanisms in systems which in bulkequilibrium display spatially homogeneous long-range orientational order in absence ofperturbations. As testbed material, we select thermotropic nematic liquid crystals. In bulk, theyexhibit weakly discontinuous order-disorder phase transformation on varying temperaturewhere the ordered nematic phase features spatially uniform axial order along an arbitrarysymmetry breaking direction. However, due to continuous symmetry breaking (CSB) theestablished order is extremely susceptible to various perturbations which are in real systems ingeneral always present. We theoretically illustrate how diverse complex patterns could beexcited. Particularly intriguing configurations could appear if topological defects are present that could be generated via CSB.

 The report presents studies in 4′-methoxybenzylidene-4-n-butylaniline (MBBA) and isooctane (Sol) mixtures, for which the monotectic-type phase diagram, with 2 types of two-phase (TP) domains, was found:  (i) for the low (TP1), and (ii) high (TP2) concentrations of isooctane. For both domains, biphasic coexistence curves’ have been discussed and parameterized. For TP2 it is related to the order parameter and diameter tests. Notable is the anomalous mean-field type behavior near the critical consolute temperature. Regarding the isotropic liquid phase, critical opalescence has been detected above both biphasic regions. For TP2 it starts ca. 20 K above the critical consolute temperature. It is classic (mean-field) above TP1 and non-classic above the TP2 domain. The long-standing problem regarding the non-critical background effect was solved to reach this result

Nano and micro-structural complexity of nematic liquid crystal configurations

Phase Equilibria and Critical Behavior in Nematogenic MBBA—Isooctane Monotectic-Type Mixtures

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LiFePO4 is an important base material for the generation of new batteries. One of the important developments is its use in the form of a solid glass, which allows an increase in the electrical conductivity after the high-pressure process. Such a treatment allows full control of the vitrification and nanocrystallization processes as well. This report shows the basic reference for the pressure dependence of the glass transition temperature. The unique behavior has been proven with a maximum of Tg(P) already at moderate pressures. The protocol for depicting the resulting evolution is as follows: it enables a reliable extrapolation beyond the experimental domain. The importance of the presented results for the general topic of glass transition physics is also remarkable due to the scant evidence of the existence of systems with clearly inverted vitrification under compression.

Evolution of Tg in lithium-based glassunder pressure

Continuous isotropic–nematic transition under pressure

Landau-de Gennes mean-field model predicts the discontinuous transition for the isotropic–nematic (I-N) transition, associated with uniaxial ordering and a quadrupolar order parameter in three dimensions. This report shows pressure-related dielectric studies for rod-like nematogenic pentylcyanobiphenyl (5CB) and its nanocolloids with titanian barium nanoparticles. The scan of the dielectric constant revealed the near-continuous I-N phase transition in the compressed nanocolloid with a tiny amount of nanoparticles (x < 0.1%). For the nematic phase in 5CB and its x = 1% nanocolloid the enormous values of the dielectric constant and the bending–type, long-range pretransitional behavior were detected. 

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Soft Matter systems have common features, such as the dominance of elements or local structures on the mesoscale, combined with their relatively weak interactions, which turns out to be sufficient to obtain a tendency to self-organize with even a small change in parameters. This additionally leads to extraordinary sensitivity to even minor endogenous and exogenous factors, e.g., nanoparticles and pressure. In the case of the latter, relatively low pressures P~1 GPa, or even much lower ones, can lead to phases/states with exotic features, often persisting after decompression.

Worth stressing, that for "classical hard matter" systems, a pressure similar to that at the Earth's core (~300 GPa) is typically required, and the resulting "exotic" properties most often disappear upon decompression.

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